1. Field of the Invention
The present invention relates to space vehicle navigation and control and more particularly to a method and apparatus for providing an accurate Global Positioning System (GPS) time reference for spacecraft such as Low Earth Orbit (LEO) satellites.
2. Prior Art
Satellites and other spacecraft require a time reference for controlling their navigation and orientation and coordinating their operations with other devices with which they interact. A common source of a time reference is the GPS which provides signals from which the local satellite clock bias can be determined to generate an accurate time reference. Typically, receivers for detecting and using GPS signals require the pick up of signals from a number of GPS satellites, for example, the Three Axis Navigation System (TANS) Vector receiver for satellites, available from TRIMBLE NAVIGATION, of Sunnyvale, Calif., requires 4 GPS satellites to be in the view of the receiver in order to provide the receiver clock bias to generate the time reference. If there are less than 4 such satellites in view, then the clock bias solution is held fixed, which results in large errors in the time reference.
3. Problem to be Solved
It is therefore a problem in the art to obtain a reliable and accurate receiver clock bias signal for generating an accurate time reference for a spacecraft or satellite.
4. Objects
It is accordingly an object of the present invention to provide a method and apparatus that will obtain a reliable and accurate receiver clock bias signal for generating an accurate time reference for a spacecraft or satellite.
It is a further object of the invention to provide a system that uses GPS signals to obtain a reliable and accurate receiver clock bias signal, which system is capable of generating an accurate time reference for a spacecraft or satellite with less than 4 GPS satellites within the view of the receiver.